Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communications of large databases, but has also enabled the real-time (and beyond) digital communications of audio and video content. This high bandwidth communication is now carried out over a variety of facilities, including telephone lines (fiber optic as well as twisted-pair), coaxial cable such as supported by cable television service providers, dedicated network cabling within an office or home location, satellite links, and wireless telephony.
Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range so as to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over “wired” networks, including the telephone network, cable network, or a dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network.
Wireless communication facilities overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled. Furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, since it is common to have multiple users within a certain frequency range, wireless telephone communications are subject to interference among the various users within the nearby area. Radio frequency data communication must also be carried out within specified frequencies, and is also vulnerable to interference from other transmissions and sources of noise. Additionally, radio frequency communications is inherently insecure. This is due to the uncontained propagation of the signals. Satellite transmission is also currently expensive, particularly for bi-directional communications (i.e., beyond the passive reception of television programming).
A relatively new technology that has been proposed for data communications is the optical wireless network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the use of an optical fiber as a waveguide. Optical wireless communications is inherently secure because in order to snoop on the transmission, the transmission would need to be broken. A broken transmission link is readily detected.
It is contemplated that the optical wireless network according to this approach will provide numerous important advantages. First, high frequency light can provide high bandwidth, for example ranging from on the order of 100 mega-bits-per-second (Mbps) to several giga-bits-per-second (Gbps), when using conventional technology. Additionally, this high bandwidth need not be shared among users, when carried out over line-of-sight optical communications between transmitters and receivers. Without other users on the link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing to permit multiple simultaneous communications. Bi-directional communication can also be readily carried out according to this technology. Finally, optical frequencies are currently not regulated, and as such no licensing is required for the deployment of such networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks between buildings. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector.
It will be apparent to those of ordinary skill in the art of the present invention that reflections of transmitted light beam from stray surfaces present in the operating environment of the optical wireless network may present themselves as a potential source of concern and compensation for such reflections is of importance to this technology. In fact, should a reflection be detected by an optical sensor of the transmitter that is the source of the transmission, it is possible for the transmitter to consider the reflection as a light beam from another transmitter and lock onto to the reflection of its own origin. Particularly for laser-generated collimated beams, which are particularly intense (i.e., high transmitted power with a small cross sectional area), reflections from stray surfaces may present many images of the transmitted signal with sufficient strength to affect system performance. Especially considering that in many contemplated applications of this technology, the transmitted light beam is transmitted through transparent plastic covers or through glass windows, many possible reflective bodies are present throughout the operating environment of the optical wireless network, providing many reflections of the transmitted light beams.
Therefore, a need exists in the art for an optical wireless link that can detect and determine reflections of its own transmitted light beam and to prevent the optical wireless link from locking onto the reflections.